DPI and PPI are pretty much the same thing. DPI refers to printing (dot per inch)and PPI refers to a computer screen (pixel per inch)
We recommend that images be at least 300 dpi for the best results, but 260 should print fine. You may want to stick with a smaller size book, like 9 x 7, if you are going to use lower resolution images.
Of course, we have customers from all over and not with the same internet service, so uploads are occasionally a problem. If you do not have the option of uploading from a location with a better internet connection, I would recommend uploading a few photos at a time, or you could zip them up and upload them that way. Here is a link to our page about zipping and uploading a folder: http://www.viovio.com/wiki/index.php?structure_id=412
I hope this helps!