If the photo is 10" x 8" at 72 dpi that means it is 720 pixels by 576 pixels. If you change it to 300 dpi it will only be about 3.33" x 2.67" which is not what the printer meant. What is wanted is a higher resolution photo to start with. If you cannot rescan the originals then you're a bit stuck.
They can be resized with software (Irfan View click here) is free and very powerful, but it won't add any more detail than is in the original. They will just become blurred.
The dpi setting can be changed on TIFFs. The printer will prefer TIFFs. Changing the resolution to 300 dpi will just mean the photo will be a quarter the size when placed at 100% in a DTP program.
Use an image editor like the MS Photo Editor (comes with MS Office)or the one in the download section of the PCAdvisor site. Open the .tif file and then "save as" with a jpeg extension. The image itself is measured in pixels not dpi (dots per inch). If you try to enlarge a low resolution image, it is likely to be pixelated - large, blurred square blocks. dpi refers to the resolution of the printer and you can set your printer to print at any resolution within its limits.
NO,,,,, if you do all it will do is make the photo bigger it will not improve the quality of the photo. You can downgrade from 300 to 75 but not the other way. If you scan a photo scan at a high setting then you can reduce if you need to send by e-mail.