Filters are used to filter image urls out or in. This depends on the filter mode.
A filter is separated with a semi-colon (;). Any url that contains the text specified in the filter will be removed.
If for example you want to include only images of cats and dogs do the following:
1. Set filter mode to skip no-matches
2. Add the following to the filter textbox: cat;dog
3. Either paste image urls from your clipboard or add them via the add form.
To use a regex, prefix the filter with an exclamation mark (!).