to attach a screenshot, in the bottom left corner of your post, click "Go Advanced", then select the image file, then click "Upload file (s)", once done it will show the files in a list, and at the bottom you can then click "Close Window", once returned to the post window you can then click to complete the post, and the image will be attached.

to add a link to google sheets file, put the link into a text file, and attach it in the same method as for an image, using "Go Advanced"