When you paste it on a page, it should say Image:Glock17.jpg. If you forget any part of the link it won't work right. Then put it in double brackets.
Since this link will make it a big image without control over the size add this after words:
[[Image:Glock17.jpg|thumb|none|400px|Glock 17 - 9mm.]]
This puts a thumb bubble over the image, controls whether it goes left, right, or nowhere (none is prefered as it doesn't screw up the page), 400px controls the size of the image (px always stays, but you can increase or decrease the size to 300px, 500px, 600px, etc.), and the final part is the caption for the image.
Hope I helped. I can't stress enough though that once you make pages, look at pages made by me, MT2008, and Speakeasy801, as they are a perfect example of how the format for page making should be on the site. This means not linking the gun page in the title, but in the description, adding a gun image so the page viewer can see clearly what the gun is said to be in the screencaps, and adding captions to the images (you don't have to add captions all the time, but none of the time kind of sucks because now you have an article with less to read, and I personally like to know what's going on if the movie is one I haven't seen).
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