And you may consider SVG at least for web use. If you must use a raster format for some (or no) reason, (in the possibly specific version) for screen use it should have the exact pixel x pixel size at which it is displayed/used. And have you used the optimization ...
{ "closeImageIconURL" : "https://community.esri.com/skins/images/13CF7416385B4D5513EB6CA9B4FBA0FF/responsive_peak/images/button_dialog_close.svg", "activecastFullscreen" : false, "dialogTitleHeadingLevel" : "2", "dropdownMenuSelector" : ".lia-menu-navigation-wrapper", "accessibility" : ...