When the Expand Image action is enabled, the image in the associated tile will expand to its actual size, if the display monitor can accommodate. If you are not comfortable with how much your image expands (you may find it a little small, or not big enough), you can resize the image, using your preferred photo editor, then re-upload it to Zoomforth.
This article that specifies the minimum size of an image, depending on the number of column grid, may give you an idea about how many pixels you may want to increase your image to.