Solution checked on PS 1.7.8.10:
1. rename folder: //img/temp to //img/temp_old
2. create a new folder //img/tmp
3. go to Back Office: Sell>Catalog>Products -> scroll down and select "Items per page:" (current maximum 300) -> page will regenerate all thumbs in tmp folder.
N.B. if you have more than 300 products > filter them per ID columns, for example: from 2 to 600 etc.
enjoy 
Solution checked on PS 1.7.8.10:
1. rename folder: /img/temp to //img/temp_old
2. create a new folder //img/tmp
3. go to Back Office: Sell>Catalog>Products -> scroll down and select "Items per page:" (current maximum 300) -> page will regenerate all thumbs in tmp folder.
N.B. if you have more than 300 products > filter them per ID columns, for example: from 2 to 600 etc.
enjoy :)
