The key thing to note here is that each image file format is best for specific purposes.
JPEG stands for “Joint Photographic Experts Group.” That right there should tell you that the .JPG file format is optimized for photographs. It doesn’t work so well for line-art, logos, gradient or tiled / patterned backgrounds, and the like. .JPG’s DCT (Discreet Cosine Transform) artifacts (the “blocky” artifacts we all know and loathe) are much more noticeable with line art and logos than with photos.
PNG has replaced GIF for everything that GIF was ever good for, except one, and that only because no clear standard has emerged: animations. Animated .GIFs are well known on the Web. There are two competing standards for animated .PNGs: APNG and MNG.
APNG is supported in most modern browsers, and is also fully backwards compatible (the file extension is .PNG, not .APNG, and if any program that can display .PNGs but doesn’t know APNG is “fed” an APNG, it will display either a stand-in image of the developer’s choice, or the first frame of the animation if no such stand-in was provided — as far as the older program is concerned, it’s just an ordinary single-frame .PNG with that one image in it — the rest is safely ignored). Gecko (FF, etc.) and Presto (Opera) support it natively, and Google Chrome (using Webkit) can with an add-on.
MNG has the backing of the actual PNG-format development team, but is its own format and is not backwards compatible, but is more powerful and flexible. Right now, only KHTML-based browsers (Konquerer) support it: not Trident (IE), Gecko, Webkit (Chrome, Chromium, Safari), nor Presto.
PNG does everything (except animations unless enhanced with APNG) that GIF does, and better. All else being equal, a .PNG will almost always be smaller than a .GIF at the same resolution and bit depth. Like .GIF, .PNG can support color depths up to 8 bits per pixel in indexed-color (paletted) mode, but unlike .GIF (yet like .JPEG) it also supports direct-color mode at 24 bits per pixel.
In either mode it can add 8 bits of alpha transparency, unlike .GIF (which can only do indexed color transparency [pick a color out of the palette to be replaced with 100% transparency, aka invisibility] — .PNG can do that, too). Alpha transparency produces much better results than indexed transparency, because the pixels can be partially transparent, whereas with indexed transparency (the only kind available in .GIF) your choices are either opaque or invisible. This makes for “halos” around non-rectangular objects when placed against background colors other than the one the .GIF or indexed .PNG was originally “matted” against. It also inhibits being able to do effects such as glows, drop shadows, and, of course, see-through colored objects (without dithering). Alpha transparency can do all of those things with ease, against almost any background (glows would be largely invisible on a white background, and drop shadows would be invisible on black, but you know what I mean).
Yes, you can do 8-bit alpha transparency in an indexed-color .PNG! And guess what? Even Microsoft Internet Explorer 6 can display those just fine, complete with transparency! It’s only the 32-bit .PNGs (24-bit RGB color + 8-bit alpha) that IE6 choked on and displayed as gray!
The most well-known program that can export PNG8 (indexed color) with alpha transparency is Adobe (formerly Macromedia) Fireworks. The Photoshop “Save for Web and Devices” (at least as of CS3) could not do that, despite having basically lifted the feature from Fireworks when Adobe bought out Macromedia. It can save PNG8, but only with indexed-color transparency.
Anyway, full 32-bit (or even 24-bit) PNGs will be pretty large, though usually much smaller than the nearest equivalent .BMP or .TGA or uncompressed .TIFF or some such (unless you’re trying to do a photograph with it — that’s what JPEG is for!). It will usually be somewhat smaller even than .RLE (losslessly compressed .BMP) or losslessly compressed .TIF, all else being equal.
Unlike most of these other formats, PNG also supports 48-bit RGB color, with optional 16 bits of alpha transparency, for extremely high quality (much higher than most monitors can display). These are best used as an intermediate storage format, to retain information from a high bit depth scanner or camera (RAW mode) or some such. Their file sizes would be quite large, despite the lossless compression.
One thing that .PNG cannot currently do is handle non-RGB color spaces such as CMYK or L*a*b.
In short:
- For photographs, use .JPG.
- For line art and logos of limited color, use indexed-color .PNG (PNG8), with alpha transparency if needed.
- For line art and logos of extensive color (e.g. lots of gradient fills, metallic chrome-type reflection effects, etc.), use direct-color .PNG (PNG24) with alpha if needed, if you want the best quality or need transparency (and don’t mind it not working in IE6 or use one of a variety of IE6 workarounds for transparent PNGs), and don’t mind the larger files and bandwidth usage. Otherwise, use .JPG, but be aware that the quality will be degraded. You may need to crank the JPEG quality up pretty high, especially for logos or other graphics with “text” in them, which would reduce your file size savings.
- For non-Flash/Silverlight/video/HTML5 Canvas animations, .GIF if the main choice at present, but be prepared to switch to APNG (I don’t think MNG is going to beat it, despite the more official support from the JPEG developers).
