Once you have selected the image you want you can save it to a file for images appropriate to your niche.

To make the image more personalized or to include some text with it you can upload it to Canva https://www.canva.com/

Once in Canva you will see a row of templates. Click on +More and you will have an extensive range of templates

Choose the one which best fits your purpose. Note the size of the template underneath it.

I am choosing Facebook Post.

On the left hand side bar there is a tab to upload your image. This is where you can insert the image you have saved from Google images. .

Put your cursor on the image and push it across to the blank template.