jpg errors in Outlook

jpg/jpeg files are corrupted when sent with Outlook 2000 and suing XP. I tried zipping them, sending to web mail – all to no avail. If I save as bmp files they are OK. The file associations are set up OK. I tried opening with PhotoShop and IE and MSPaint and none work. The porblem is also somewhat sporadic as some pictures “get through” just fine.