Weeeeel, since the 1680x1050 image has a very weird aspect ratio, you actually won't be able to get a 1600x1200 crop from it without upsampling (making the image larger). Zenphoto won't do this as it degrades the quality of the image considerably.
You can make any size less than 1680x1050 though.
`i.php?a=albumname&i=image.jpg&s=1228
&cw=1024&ch=768`
I had to calculate that 1228 width by hand... so I see why you need to be able to specify height. Otherwise it's just a matter of making it wide enough so it's high enough to get the crop you want. The final image size is going to be cropped from this aspect ratio original, so use cw and ch to specify the crop. You can also fine-tune the position with cx and cy.
Bottom line though, if you want it to resize up to 1600x1200, I won't put that in the official release. You have to remove a few lines in i.php... just comment out the if statement after this comment:
`// If the requested image is the same size or smaller than the original, redirect to it.` and that'll let you make images bigger than the original. Be warned though, you're losing quality...
I'll add the ability to specify height or width directly soon, until then calculate the s value needed from trial and error or the aspect ratio.
[b]Edit:[/b] It's also important to see that you don't need to generate [i]image files[/i] beforehand at all. The image doesn't have to be generated to be viewed, and it's all done on the fly, so using this i.php?... stuff is exactly the same as using a real image's URL. You can just throw around that URL like an image; put it in <img src="" /> tags and everything.
So I hope you're not confusing the image generation with other programs' methods... it's a lot slicker than that.