Showit automatically optimizes your images but we do have some tips for sizing to make sure we have a high enough quality file, while also making your upload process fast.
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Images formats supported: JPG, PNG, GIF
Color Space: sRGB

We suggest resizing your original sized images to 3500px on the long edge to make the uploading process faster. Original sized files are also accepted but will take longer to upload. The DPI resolution does not matter but you can set 72 if it's necessary to set.  The color profile should be sRGB.

Our image robots 🤖 will automatically deliver resized versions to a viewer based on what device they're viewing your site with. 📱🖥

There is no need to use special compression tools to optimize your image because our servers will generate additional sizes and the extra compression on the uploaded version may result in additional artifacts in the resized versions.
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First, we only suggest using PNG when transparency is necessary and GIF when animation is used, all other images should be JPG format. Our servers do not resize PNG or GIF files, so we suggest you size them closest to the size they will be used in your design. That will ensure that you're not loading extremely large files since both of these formats can grow large very fast.

Pro Tip: as a best practice it's a good idea not to include symbols such as "+" in your image file names, for example - jim+jane.jpg
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Your blog manages it's images separately through Wordpress. We suggest resizing your images to be 100 - 200 pixels wider than the Post Content area on the Desktop layout of your Single Post template. Of course test a few images at this size on you blog to see if they meet your quality standards before sizing all images.  Using a web image optimization tool on your blog images is definitely suggested, especially if your blog posts typically contain a lot of images.

You can find more info on optimizing your blog images in this article here.