Final? version: 2.1.1
Minor bugfixes
Added localizations: Now you can switch between English and Russian

Changed manifest to version 2: Now users don't need to manually allow this extension to access data on all websites, this feature is built-in in manifest verison 2.
Changed slider values behavior: Now, if you input maximum value, that is lower than minimum value and vice-versa, the respective value will change accordingly.
Minor code improvements.

Fixed version numeration: From now on, versions will be numerated as 2.x.x.
Better interface: Cool dark and modern style using pure CSS.
Options feature: You can set permanent minimum and maximum image size for downloading, these options are saved on local level (they'll remain unless you delete your browser's files).
Helpful messages: This extension requires enabled data access on all websites to download multiple images, so it's now declared in extension's interface.
Fixed minor issues: Fixed issue, when "Minimum\Maximum size of downloaded pictures" text was becoming "null", if you manually input size.

Prettier look.

Extension now can work in the background.