From YouPic terms & conditions
Content submitted Or Made Available For Inclusion On The Service
1. You acknowledge that:
1.2. The license granted to YouPic includes the right to use your Content fully or partially for promotional reasons and to distribute and redistribute your Content to other parties, web-sites, applications, and other entities, provided such Content is attributed to you in accordance with the credits (i.e. username, profile picture, photo title, descriptions, tags, and other accompanying information) if any and as appropriate, all as submitted to YouPic by you;
From 500px terms & conditions
CONTENT SUBMITTED OR MADE AVAILABLE FOR INCLUSION ON THE SERVICE
By posting Content to the Site you hereby grant to 500px a non-exclusive, transferable, fully paid, worldwide license (with the right to sublicense) to use, distribute, reproduce, modify, adapt, publicly perform and publicly display such Content in connection with the Services. This license will exist for the period during which the Content is posted on the Site and will automatically terminate upon the removal of the Content from the Site;
The license granted to 500px includes the right to use your Content fully or partially for promotional reasons and to distribute and redistribute your Content to other parties, web-sites, applications, and other entities, provided such Content is attributed to you in accordance with the credits (i.e. username, profile picture, photo title, descriptions, tags, and other accompanying information) if any and as appropriate, all as submitted to 500px by you;
There's not a lot of difference between the two. Worded slightly differently, but both retain the right to use your content for promotional reasons. Maybe you could point out the specific problem, 'cos I don't see it.