Change the frontpage post creation in wordpress that user can paste a link without a .jpg/.png for featured image

Hey, on my wordpress page (theme) i have a frontend post creation. At the moment user can paste an link with an .jpg or .png for example [login to view URL] Your job is to change it that the user can paste a shop link or a social network link and the page ask the user which picture from the webpage should be the featured image for example he paste; [login to view URL] or http://www.amazon.de/gp/product/B00CTV13Z4/ref=s9_pop_gw_g424_i3/280-7239042-3427229?pf_rd_m=A3JWKAKR8XB7XF&pf_rd_s=center-4&pf_rd_r=1WC82M8J3AC2612K6WPV&pf_rd_t=101&pf_rd_p=171793527&pf_rd_i=301128 At the moment it didn't work because there is no .jpg/.png ending on the link. If you have question. Feel free to ask