Yes, you can do it with Custom code. Here is a sample code for you:
.sp-custom-login #login .login-bg-img {
background-image: url("localhost/unei_quickstart_j3/images/2022/03/01/_spmedia_thumbs/footer-bg.jpg") !important;
}
You need to upload your image in the media and then use the full image path link within the above code.
And sign in text can be changed with the help of language override, but you will find the language phrase within the file "en-GB.tpl_unei.ini" too! Here is the path:
\language\en-GB\en-GB.tpl_unei.ini
Please open the file with an text editor and replace the text "Sign In" with your desired one. I have attached a screenshot for your ease:
https://prnt.sc/BdTbh1ONFTG9
Best Regards