Click on the export button to export your images set.
NB: It is important to export the images from ESC, and not use the original ones. The output images generated by ESC are reworked to fit your signature.
Simply drag and drop your images
Make sure that your images can be accessed by anyone
Once set, just press Upload.
Once back in your stream, select an image.
Hover "Original", Right click (or ctrl+left click), then select "Copy Link"
Go back to ESC.
Paste your image link in the proper row.
Do this for all your images.
When you did the last step, for all your images, then you are done.
Now Just Press Copy and paste your signature in your Email client.
More email clients tutorial here