Can not reduce size of an inserted image in Numbers
Image Gallery is a special tool that allows more than one image to occupy the same space while allowing the user to view all of those images one by one. An image inserted via the Image Gallery into Numbers can not be edited as freely as other images. See this short video if you wish to learn more about how to better use Image Gallery (the video is for Image Gallery in Pages but its functionality is the same in Numbers.)

If you simply want to use a picture in a Numbers table resizing it freely, either copy and paste it from Finder or click on the Media on the toolbar at the top of Numbers and then click on Choose and then find and select the picture you wish to insert. 

You can also drag and drop any compatible picture from Finder into a Numbers cell and Numbers will automatically resize the picture to fit the cell that it is dragged into.