Make assets folder into your document root.
cd /path/to/document/root mkdir assets chmod a+w assets
Copy plugin files to frog/plugins/assets/ folder. If you do not have Git use download link instead.
cd frog/plugins/ git clone git://github.com/tuupola/frog_assets.git assets
cd frog/plugins/ git clone git://github.com/naehrstoff/image_resize.git image_resize
cd frog/plugins/ git clone git://github.com/tuupola/frog_jquery.git jquery
cd frog/plugins/ git clone git://github.com/tuupola/frog_jquery_ui.git jquery_ui
Go to admin and enable Assets and Image Resize, jQuery and jQuery UI plugins.
NOTE! If you are using Page not found plugin disable and then re-enable it. This makes it run as last plugin on the page and fixes compatibility issue with Image Resize
Now you are set to go.
Assets tab includes simple interface for uploading images or other files. You can delete an asset by dragging it to trashcan.
When editing a page you can drag an asset to textarea. Full URL to asset will appear.
By default files are uploaded to assets folder. If you need more folders you can create them from Assets plugin settings.
NOTE! When adding a folder plugin tries to create it for you. If it fails you will be notified and you must create the folder manually. Also when you remove a folder from settings it will NOT be deleted from server automatically.