Why background color is changed when hovering on .gif images?

Question

But this issue doesn't exist for .jpg images,

why?

Answer 1

The reason most likely has to do with the fact that GIFs support transparency, so if you have a GIF inside an <a> element and the stylesheet changes the background colors of your <a> elements, the color change will be seen through the transparent portions of the GIF.

To change this, you can modify your stylesheet to not change the background color of the anchors used for your images, or you can resave your GIF file without transparency.