There’s a few ways to do this.
There’s no need to delete the one that’s there. Changing artwork always replaces the previous one.
You can copy one and select>paste to multiple others (ctrl c > ctrl v) directly in those fields.
You can select the files you want to change and drag-n-drop an image directly from Win Explorer or your web browser. The file must be dropped onto the bottom area, underneath the Summary, Info, Misicbrainz, Sorting, etc tabs. You also don’t have to select the Artwork field, it can be any field and Jaikoz will understand it’s artwork for that track.
You can also double click the little number next to the artwork field to open up a dialog. This is where you can add more artwork files (alt covers, back, liner notes, etc). Since you can only edit one at a time this way, you’ll have to copy>paste the rest of the album.
BTW, ctrl-v pastes all values in multi-value fields, ctrl-shift-v pastes only the first values. This applies to all fields that have a little number next to them.