image sizes constrained?

i recently installed a new theme. i had the designers expand the post area to 750px, because the post area of the theme was originally much smaller, and i wanted room to add larger photos. yesterday i went to post my first post with the new theme…and i cannot insert photos wider than 650px (which is what i am assuming the original post width of the theme was). the words take up the 750px post width just fine. but i can’t add pictures of that size, nor can i center them within the 750px post width (which wouldn’t be what i would choose, anyway, but it would at least make the post look a bit less ridiculous). media size settings are at 750px, as they should be. i deactivated all my plugins one by one in order to see if that was causing a problem. nope. i think it has to be something in the theme.

i’m really frustrated. i have combed through my css stylesheet to try to find a value i could change to fix this, but i’m certainly not a coding expert, and i couldn’t find anything. i contacted my theme developers about the issue, but i haven’t heard back from them yet. in the meantime, could anyone offer some code to plug into the css editor that might correct this problem? i wanted my new post to be up yesterday. ??

thank you so much for reading through this, and for any and all help. i appreciate it greatly. <3