Find the image on your computer. Images should be JPEGs or GIFs, not Bitmaps because they take up too much space.
Give a summary in the "Summary" field.
Find or create an appropriate template discussing the image's copyright status, or give the copyright status in the summary. All screenshots should have the {{Episode-screenshot}} template.
Click "Upload File"
Wait for it to finish uploading.
Link to the image on all approprate pages. See below.