Hi Rahul,

Greetings to you!

According to Salesforce knowledge article: https://help.salesforce.com/articleView?id=000086448&type=1

Salesforce Classic

Upload an image into the Documents tab
1. Click the Documents tab, or the '+' icon to view all tabs and choose Documents from there.
2. Click New.
3. Fill-up the required fields and make sure to check the Externally Available Image checkbox.
4. Select which folder you'd like to save the image to.
5. Select the 'Enter the path of the file or click browse to find the file' radio button.
6. Click on the Choose File button and navigate to where the image is located on your computer.
7. Click Save.
 
Get the URL of the uploaded image
1. In the Documents tab, click on the newly uploaded image.
2. Right-click on the preview pane of the image you uploaded and click on 'Copy image address.'
 
Add the image to your email signature
1. Click on your name at the upper-right portion of the screen.
2. Click My Settings.
3. In the Quick Find search box on the left-side of the screen, type-in 'My Email Settings.'
4. Click on My Email Settings.
5. In the Email Signature field, type-in <img src = "link here"/>.
6. Click Save.
 
 
Lightning Experience
 
Upload an image into Files tab
1. Click the Files tab, or the App Launcher icon to view all apps / tabs and choose Files from there.
2. Click on the Upload Files button and navigate to where the image is located on your computer.
 
Get the URL of the uploaded image
1. In the Files tab, click on the newly uploaded image.
2. Right-click on the preview pane of the image you uploaded and click on 'Copy image address.'
 
Add the image to your email signature
1. Click on your avatar at the upper-right portion of the screen.
2. Click Settings.
3. In the Quick Find search box on the left-side of the screen, type-in 'My Email Settings.'
4. Click on My Email Settings.
5. In the Email Signature field, type-in <img src = "link here"/>.
6. Click Save.

It's important to note that this will only work when sending emails in HTML format and does not work if a template is selected, unless the signature logo is included in the template.

If an email is sent in plain text format it will display <img src = "link here"/> in the signature. If you're utilizing a template you'll need to include the image and signature within the template's body.

I hope it helps you.

Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in the future. It will help to keep this community clean.

Thanks and Regards,
Khan Anas