Is there a way for friends to upload files on my Dropbox folder without installing dropbox?

Ask them to upload by E-mail. You get a unique email address and any files your friends send to this address ( as attachments) are automatically uploaded to your DropBox account. Complete instructions here : http://www.labnol.org/software/upload-dropbox-files-by-email/18526/

You can sync your dropbox with this (http://www.dropitto.me/) and then provide them with a password, and then they can load files straight into your dropbox.

There are some services that does what you need but I like a simple interface and a simple approach.  I use http://dbinbox.com/ . Whats nice in using this, is the simplicity and drag and drop option.  Its also tells you exactly whats its doing while you wait.  No guessing what to do. It also has a text message option that you can give a message if you like

Are these photos on a mobile device? If so they can use mms or use multi platform app such as liveshare and from there you can drop it into drop box. I agree with Ramishka that Email would be easiest route.

We have created a file upload service, named http://CloudWok.com, with which you can generate an upload page on which users can upload files that are then directly transferred to your Dropbox (we also support Google Drive, Box, S3, and other cloud storage services). Thereby, you can use CloudWok to collect files from other people who do not need to have a Dropbox account. You can also protect the upload page with a password in case that you want to add extra security. When files are uploaded, you are notified via e-mail.

Dropbox now has a native feature called “File Requests” that allows you to issue a single-use upload ticket to someone who doesn’t have a Dropbox account, which will allow them to upload a file directly to your dropbox.