Image Uploading Suddenly Glitchy/Errors

I haven’t had trouble in the past with image uploads, and this happened out of the blue – I’ve not made any changes to WordPress or otherwise on my computer in a while…

Now, most of the time when I try to upload, especially any image larger than “blog size” (about 300 pixels across), I often get an “HTTP Error” and it won’t upload at all… Or else it gets the full-size image through, but doesn’t have the sizing options and such, but instead just rolls off this long string of error text:

Warning: imagecreatefromstring() [function.imagecreatefromstring]: gd-jpeg: JPEG library reports unrecoverable error: in /homepages/24/d274473209/htdocs/wp-includes/media.php on line 254

Warning: imagecreatefromstring() [function.imagecreatefromstring]: Passed data is not in 'JPEG' format in /homepages/24/d274473209/htdocs/wp-includes/media.php on line 254

Warning: imagecreatefromstring() [function.imagecreatefromstring]: Couldn't create GD Image Stream out of Data in /homepages/24/d274473209/htdocs/wp-includes/media.php on line 254

I know enough to do my ungrades and such, but I’m not at all savvy about such things… Any ideas??

Thanks!