Set Image Bounds
This tool will make it easy to override the global image bounds set in Settings/Images/Image bounds on a per image basis.

To use it select an images thumbnail and then use the 'more' or right click to open the contextual menu and then select 'Set Image bounds…'.
The settings window will open.
The default is to just adjust the image bounds for slide images, to adjust thumbnail bounds check one or both of the checkboxes.

Use the 'Up' or 'Down' arrows to adjust the maximum width/height or click into the number field and manually enter a value within the limits shown.

When happy just click 'ok' and those image bounds will be applied for the slecected image.

You can see the effect of setting the new bounds in 'Edit' mode, look at the variables pane and you will see two new variables for max image width/height and the size as entered for the 'value'.

To remove the created user variables just uncheck the appropriate checkboxe.