Try to use png images and try to downsize them before you use them in x5, this saves time in x5 and ensures best loading time/size handling. I keep images for internet max 1000px width.

For shop it is a pop up so I downsize even till 500px max. I use Irfanview for this, where you can also set the dpi (lower is lower kwality and also smaller size of picture, you can also choose to let the program pick the best dpi so you always get a good quality picture for internet with the smallest size.