error when i try to change image

You may need to upload the image to another ap to change the file name to gif, png, jpg etc. Lunapic or Paint will work. After doing that, you can import the file to your homestead file manager.

ADDING IMAGES SB+:
Here is an article to assist with uploading images to Sitebuilder. https://community.homestead.com/homestead/topics/sitebuilder-plus-add-an-image
Select insert > image. If you add a photo gallery, it will auto thumbnail the image(s)
You can hold down the shift key and click on multiple images to upload more that one image at a time.