Small Bug/Improvement

Hi,

first of all, thank you for this plugin!
Especially the use of the WordPress get_template_part function is something that makes this plugin really useful for customization.

A small bug/improvement:
When adding a new image widget, it is only possible to choose from the wordpress standard image sizes. Only when saving the image and reloading the page, all available image sizes can be chosen. This is because you retrieve the image sizes from the selected image. I thinkg it would be a better idea to just use all registered image sizes, even though there might be images that do not have a certain image size (e.g. if they were added before the image size was registered).

Best,
Mario

https://www.ads-software.com/plugins/simple-image-widget/