this is almost always a browser problem and not a filesize or network or internet speed problem.
trad gang has unlimited file space and bandwidth. when you upload an attached image from your computer to trad gang it's going to be sent to the trad gang server, but the vehicle that sends is your browser. large images can time out the browser, particularly if your browser is competing with a lot of users logged into trad gang. making the image file smaller will almost insure it will upload fast.
what's a good image size? 1000 to 1500 pixels wide is a good rule of thumb. this is about 1/4 the size of most cellphone and camera images. for the very most part, 800 wide is more than enuf.