This topic is specifically approved for APUG - it is the only "how-to" scanning topic that is.

For those who have larger prints that won't scan in one pass on their scanners, there actually is a useful free, Microsoft product that does a good job at stitching images.

It is called Microsoft Research Image Composite Editor (or Microsoft ICE) and can be found here:

Did you ever expect to see the words "useful free, Microsoft product" together?

And with respect to Umut's initial question, I would suggest you look carefully at the installation guide for gallery uploads - particularly the size limits.

I'm slowly discovering some of the complexities of re-sizing for the web. Certainly it seems to help a bit to re-size in stages, with close examination and some sharpening between each step.