http://pixlr.com/editor allows users to quickly and easily convert their logos from JPEG to PNG.
 
To do so, take the following steps:
 
Open the link and select ‘Open image from computer’;
 

 
Select the logo you want;
 

 
Then, click the ‘File’ tab and select ‘Save’;
 

 
In the format options, select PNG and click ‘OK’;
 

 
Now you have your logo in PNG format;