sometimes either the file is deleted, or, well your browser is being a turd. Right click -> view image, show image etc.
Now I hope you know the resons why they want you to read the forum information before posting also try reading the faq and support, system thats been there a for a long time you should check it out as a new member.
'user posted image' is the default alt text for images on IPB. Because of that, whenever an image cannot be loaded, it displays 'user posted image'. This could be due to the image being deleted, moved, or being protected from hotlinking. If its being hotlink protected, you can right click and hit view image because that bypasses the method hotlink protection uses (refferers). Alternatively, the right click> view image method will work because the image simply didn't load correctly, in which even a page refresh will fix the issue. View image wont work if the image has been moved or deleted though.