For the Image Plus App, here's our short tutorial video.
For the Static Html App, go ahead and click on "Upload Image" and upload your image. To use these images on your tabs, scroll over the image on your tab above the editor boxes - the image will slide out and code will appear, copy and paste this code the editor to use the image. The image code will look like this.
To change the tab image of your tab, see here for instructions.
To edit the dimensions (crop, resize), go ahead and check out check out picmonkey.com.
To add text to an image, check out canva.com.