Although this is an old thread, I've found a way to compress the images by using the Get File Thumbnail action connector.
You can use the "Get File Thumbnail" to generate a URL Dynamic content.
From here you can use the "Upload file from URL" action to generate a smaller down-sized image.
You can then get the file content from the uploaded file and do any other actions you require.
Although there's some limitations with specifying the exact size you want, looks like you can choose only a few specific thumbnail sizes.
Using this you should be able to downsize most images to a manageable size without premium actions or third party connectors.
