Select the paper clip. Select photos. Select the app (just once or always, your choice). Select the first picture.
Repeat for each picture.
If the total size of the pictures you're trying to send is too large, that's it, you can't send any more. How many you can send depends on their size. (Each carrier has a maximum MMS size . Verizon limits you to 1.2MB, AT&T appears to limit you to 600KB [they don't say there's a limit, they just "reserve the right to" compress photos to keep them under 600KB], etc.)
If you have to send someone a lot of larger pictures, and you don't want them to be compressed beyone all usability, upload them to a filesharing or picture sharing (or cloud) server and text the person the addresses of the pictures. Mega has a maximum size of at least 2GB (I've downloaded 2+GB files from them.)
09-28-2014 03:23 PM