According to Success Criterion 1.1.1 (Non-text Content), all non-text content should contain at least a short text alternative, such as through alt text. However, if the short text alternative is not enough to fully describe the non-text content, a long description should be provided. An example may be the contents of a graph: the alt text may say "Graph of sales by quarter" but it does not describe the points of the graph.
This demonstrates the correct usage of placing alt text on an image submit button. Since the purpose of the alt text is to provide a description of the action being performed, the alt text should reflect that goal, rather than being the usual description of the image itself.