An announcement image works when people can quickly tell what matters. A main title, a short supporting line, and a small amount of essential context are easier to read than a full page of text squeezed into an image. Begin in your design application, where you can change type size, spacing, contrast, and the amount of copy.
This page starts with PNG at the source dimensions, sharpening off, and no crop. Supply a finished raster design. The tool does not add text, change fonts, lay out a poster, or convert an image back into editable text. A screenshot of a tiny graphic is not a substitute for exporting again from the original design file.
Keep important information away from the outer boundaries, particularly if the same artwork will be reused in a different layout. A feed image, event cover, and Story have different shapes and can be previewed differently. Instead of forcing one dense design into all of them, make separate source layouts where needed. The compressor can prepare each finished layout, but it cannot make a wide event banner and a vertical Story share a useful composition automatically.