First, you'd have to visit your login page in the browser of your choice. Once there, click in the address bar... you'll see what's referred to as a Favicon at the left of the address. This is a small corporate icon. Drag that icon to your desktop... it should create a .webloc (web location) doc with a generic icon.
Next, do a Google search for the People's United Bank logo (or simply save my shared pic, if it's the right one).
Now comes the fun part! Select, but don't double click, the web shortcut you copied to your desktop. Hold down the Command-I keys to bring up the Info window... you can also do this by going to selecting the file, going to File>Get Info. In the window that pops up, you'll see the same generic icon in the upper left of the window. Click on that, and you'll see it highlighted in blue.
The next step might seem tricky, but it's really not. Open your PUN logo file in Preview, choose Select All under the Edit drop down menu, and copy the pic. Then go BACK to your Get Info window and simply paste the graphic in place of the generic icon.
https://support.apple.com/kb/PH19073?locale=en_US