It surprised me at first, when these things first came out, how quickly the industry and our clients were willing to accept crappy, low-resolution digital photos.
Years ago, polaroids were the fashion, they were black & white, but usually had decent resolution. Then color polaroids came out and clients started refusing to accept black & white photos. Then came the advent of one-hour processing and polaroids were not allowed by a lot of clients.
During the short-lived one-hour era, we tried to always get the house address numbers in the photo and it was fairly common for underwriters to ask if a photographed comp was really correct if a tree branch obliterated one of the address numbers. With these low resolution digital photos, you couldn't even start to see the address (or many other details of materials and finish). Yet, for the convenience of e-mail, these things are no longer important to our clients.
for a photo that is 1024 x 768 pixels, at "basic" image quality the file size will be approximately 100 KB and transfer time at 28.8 Kbps will be about 40 seconds. For a photo that is 640 x 480 pixels at basic quality, file size will be about 50 KB and transfer time about 20 seconds. I prefer to use the largest file size I can get away with for the sake of quality. But, if you are transmitting over a dial-up, anything larger than XGA will slow you down considerably. File size, and disk space is a non-issue as far as I'm concerned - hard disk storage is cheap these days and you can always store your old photos on CD's.
For you photo buffs or those appraisers who want a good printed image on the cover of your narrative, size matters. For the printed image, file sizes so small as those that can be sent by e-mail are a disaster. It takes UXGA or 1600 x 1200 to print a just decent photo at 8" x 6" and if you want to print at 13" x 10" you'd better have a size of 2560 x 1920 pixels and "fine" quality.