The responsive nature of the theme does have a big impact on images. Generally images fill space, up to their max size. The theme actually strips dimensions from img tags to achieve this.
The themeloom widgets certainly depend on this to work responsively with the various options.
For images used in say a text widget, you will need to make them the correct size to avoid resizing. Though they will shrink if user browser resizes to smaller window – that’s why it is responsive.
The actual sizes used by the theme are documented in user guide. These are chosen as appropriate sizes for max case.