I am a U.S. Army photographer and writer (civilian) by trade. I use digital cameras and work with digital photos every day.
I suggest that you send the photos as the following:
1. Jpg format, TIFF is simply too large for downloading. Other formats are not as common as jpg, which is an industry standard.
2. 4X5 inches or smaller. This size depends on the subject, of course. A photo of a giraffe's neck might be 5 inches vertically and only 2 inches horizontally. Or a sunset may be 5 inches horizontally and 2 inches vertically.
I would not exceed 4X6 inches, if possible, and would prefer a smaller photo.
3. Resolution of 110 dots per inch, no more. At 4X5 inches, the human eye cannot hardly discern the difference between 110 dpi and 300 dpi. However, the download time for 300 dpi is much longer.
Also, most computer screens are 72 to 96 dpi, so any more than that is unseen and wasted.
However, I send customers photos at 110 dpi in case they wish to print them on their inkjet printer.
A photo printed at 72 dpi will look unfocused. At 110, it will look fine (provided it is no larger than 4X5 inches.
If you wish to print a photo, use the highest quality (usually listed as "Best" as opposed to "Good" or "Better".
Printing a photo on regular inkjet papper at 4X5 inches will work fine, but if you want to frame that photo you'll want to print it on photographic paper.
If you want an image larger than 4X5 or 4X6 inches you'll likely want to contact the sender and have him resend in a larger size at 150 dpi or thereabouts. For photos with a lot of fine detail, such as leaves on a tree, you'll want to get it in 300 dpi if you want to make it larger, print it on photographic paper, and frame it.
But for looking at a photo on the computer, stick with 4X5 inches at 110 dpi.
I never use pixels as a determinant of photo size because the customer may wish to frame my photo.
And try going into a shop and asking for a 1200 X 1600 pixel frame!