There was a recent software change on the forum that resizes photos automatically but I don't know what the limits are. In any case in the "Photos" application select the photo, File/Export and then you can change the size before exporting.
You should be able to just drag and drop images from your Mac right into the comment editing box. Alternatively, you can use the "insert image" function in the editor's tool bar.
The resizing feature shouldn't affect this. The sizing limits are 800 pixels in either direction. However, note that if you upload an oversized image in portrait orientation, it will appear rotated 90 degrees in the post. This a quirk of the image resizing feature--you can avoid it by resizing your own photos before uploading.
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The resizing feature shouldn't affect this. The sizing limits are 800 pixels in either direction. However, note that if you upload an oversized image in portrait orientation, it will appear rotated 90 degrees in the post. This a quirk of the image resizing feature--you can avoid it by resizing your own photos before uploading.