You basically need a graphics package to do this.
Paint Shop Pro is probably the industry standard, though for straightforward
tasks, IrfanView or The Gimp are downloadable for free and are adequate for
most purposes.
Once you've got your image loaded in a suitable package, you might need to
first resize (resample) it if it's larger than necessary. This might be all
that's needed. If you need to make the file even smaller, then you need to
apply a degree of 'lossy' JPEG compression to it. For achieving the smallest
files, the JPEG file format is best. (although it may not be the best where
very high quality is needed)
Hope this give you some hints, reading-up on the subject of graphics
manuipulation will help. There is a knack to achieving a good-looking result
whilst keeping the file a reasonable size, and this takes some practice with
the software.
