Gmail. Click Copy signature here. In Gmail click the gear at the top right, then See all settings. On the General tab scroll to Signature and click Create new, give the signature a name, click in the editing box and paste. Underneath, choose it for new emails and for replies. Scroll to the bottom and click Save changes. Gmail keeps the layout, colours and links; the photo and logo must come from a hosted address (the field under each upload box), because Gmail removes images embedded in the signature itself. Gmail also limits a signature to about ten thousand characters, which a signature with hosted images is well within.
Outlook on the web and the new Outlook. Click the gear, then Accounts and Signatures (older layouts: Mail, Compose and reply). Click New signature, name it, paste into the editor and save. Choose it as the default for new messages and for replies and forwards. Embedded images are usually kept; a hosted address is safer if your organisation's settings strip them.
Classic Outlook for Windows. Open File, Options, Mail, and click Signatures. Click New, name the signature, click in the editing box and paste, then assign it to your account for new messages and replies. Outlook may add a little extra spacing between lines; the Compact and Horizontal layouts are the most forgiving. If the paste loses its formatting, close the dialog, copy the signature again and paste with Ctrl+V rather than from the right-click menu.
Apple Mail on a Mac. Open Mail, then Settings (or Preferences) and the Signatures tab. Pick the account on the left, click the plus button, give the signature a name and paste into the box on the right. Untick "Always match my default message font" so your chosen font and colours are kept. Drag the signature onto the account if it does not appear there, and choose it as the default at the bottom. Apple Mail shows embedded images, so the uploaded photo and logo work as they are.
Mail on iPhone and iPad. The Mail app's signature box is plain text and does not accept a formatted signature from the clipboard. Send yourself an email from a computer with the signature in it, open it on the phone, select the signature, copy it, and paste it into Settings, Mail, Signature; most of the formatting survives. Alternatively keep a plain-text version on the phone and the formatted one on the computer.
Thunderbird. Open Account Settings, select the account and tick "Use HTML". Paste the copied signature into the Signature text box, or click Copy HTML here and paste the code instead; both work. Thunderbird keeps embedded images and hosted ones alike.
Help desk and marketing systems. Many support, sales and newsletter tools have a signature field that asks for HTML. Click Copy HTML and paste the code. Where only an image is accepted, Download PNG gives a sharp picture of the signature; remember that an image signature has no clickable links and no selectable text.
Testing. After installing, send a message to yourself and to a friend who uses a different mail program, and open it on a phone as well. Check that the links work, the images show and the lines wrap where you expect. If something is off, change the layout or the sizes here, copy again and replace the signature; the saved copy in this browser makes a second round quick.