sounds like you have hit the limit for your upload space. there are three ways deal with this.
1) go into your user cp, under the the manage attachments button, and delete some of your older attachments to make room for new ones.
2) Become a supporting member. this gives you 10 times the upload space as a standard membership(among other benefits). you can do this here: Becoming a supporting member
3) the third option is to start using a photo hosting site like photobucket or imageshack.
here is a general how to in that department: How to post a picture
if you are doing most of your forum surfing via the app on a mobile device, photobucket has an app that is supposed to work very well with posting from phones. ive personally not used it yet, but i have been hearing good things from those i know who have.