The smaller/faster the better. <g>
Perhaps preload key images on boring text filled pages which do not have the image visible (either with jscript or 1x1 image tag size commands, after all the text) - that way when a user moves onto a busy page they will get the image poping up. That is if a user first reads this particular page.
Use common images where possible so that the users local cache loads the image.
I would base decisions of a 33 or 56 modem as I would hope that most dial up users have moved on (most of the time my 56 dialup runs right between and up to these limits).
A basic rule of thumb from the 'old days' of the web as around 30kb for a single image, but when taken into account with the page and other images and overall network activity and other variables - who can say?
When checking things locally, puttin the site on a floppy disk and flushing all caches and then reading from the floppy can help simulate an internet connection, but the transfer rates are obviously different.