Image hyperlink extending way beyond image

Hi. I am editing a wordpress theme and have run into a problem. When I post an image as a thumbnail (i.e. 150 x 150) with a link to the full size image, the hyperlink (clickable area on the image) extends out way beyond the image, like to around 400px wide. When I make the post I have all the thumbnails next to each other on the same line but when it’s published, because the hyperlink is so wide, the next thumbnail is below the first one and so on so that it’s like a column of images instead of a row. The thumbnail looks as it should, but if you put the mouse to the right of the image and click anywhere in that area itll take you to the full size image.

Where in my css template is the code to change this behavior? I can’t seem to find it. I want the hyperlink to be the size of the image and no larger.